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WASHINGTON (Reuters) - Iridium Satellite said Thursday it had no advance warning of an impending collision between one of its communications satellites and a defunct Russian military satellite above Siberia.

Amid questions of liability, negligence and possible lawsuits, the closely held company rejected suggestions that it might have come to disregard "conjunction reports" -- potential accident alerts -- routinely relayed by the U.S. military.

"Iridium didn't have information prior to the collision to know that the collision would occur," said Liz DeCastro, a company spokeswoman. "If the organizations that monitor space had that information available, we are confident they would have shared it with us."

She was responding to questions about an 18-month-old presentation by retired U.S. Air Force General John Campbell, Iridium's executive vice president for government programs.

Iridium had been receiving a weekly average of 400 conjunction reports from the U.S. Strategic Command's Joint Space Operations Centre that tracks debris in space, Campbell told a June 2007 forum hosted by the George C. Marshall Institute, a Washington research group.

"So the ability actually to do anything with all the information is pretty limited," he said, describing a kind of data overload. The conjunction reports were issued every time a potential threat object was to pass within five kilometres (3 miles) of a commercial satellite, he said.

"Even if we had a report of an impending direct collision, the error would be such that we might manoeuvre into a collision as well as move away from one," he told the panel.

Campbell then endorsed the so-called "Big Sky" theory -- that space is so vast that the chances of a collision are infinitesimal, despite more than 18,000 pieces of orbiting junk big enough to track.

"We figure that the risk of a collision on any individual conjunction is about 1 in 50 million," he said at the time, adding: "Clearly that risk is something bigger than zero."

'DODGEBALL' TO AVOID DEBRIS

Marine Corps General James Cartwright, vice chairman of the Joint Chiefs of Staff and a former head of the command that runs U.S. military space operations, said countries with satellites in space will have to play "dodgeball" for decades to avoid debris from the collision. It occurred about 485 miles

(780 km) above the Russian Arctic on Tuesday.

James Lewis, a former official at the State and Commerce departments now at the Centre for Strategic and International Studies, raised a question at the same forum about whether Iridium might have a case against the Russians.

"There was negligence somewhere," he said. Asked about this at the forum, Cartwright declined to discuss it but said he would like to see more information-sharing on debris avoidance with Russia, China, France and other countries using space.

The mishap marked the first time two intact spacecraft accidentally ran into each other, Nicholas Johnson, chief scientist of NASA's Orbital Debris Program Office at the Johnson Space Centre in Houston, told Space.com.

There have been four other cases in which space objects have collided accidentally in orbit, NASA said. But those were considered minor events and involved parts of spent rockets or small satellites.

Cartwright, who from 2004 to 2007 headed the Pentagon's Strategic Command responsible for space operations, said the military had been alerted by Iridium to the sudden "non-reporting" of the destroyed craft.

Iridium runs a network that uses 66 satellites to provide voice and data services for areas not served by ground-based communications networks. The network has about 300,000 clients.

Bryan Whitman, a Pentagon spokesman, said it was not possible for the U.S. military to track and predict the movements of all 18,000 objects in space all the time.

"Because there is so much, you have to prioritize what you're looking at," he said. "There are limits on your ability to track and compute every piece..."

"We did not predict this collision," he said.

China added significantly to space debris when it used a ground-based ballistic missile to blow apart an obsolete weather satellite in a January 2007 arms test. The United States used a missile from a Navy warship to explode a tank of toxic fuel on a crippled U.S. spy satellite last February.

China's anti-satellite test "alone increased our risk due to space junk by a factor of about three and increased the overall risk of collision by about 15 percent," Campbell told the forum in 2007.